Dideoxy nucleoside triphosphates (ddNTPs) lack an
-OH on the 3'-C as well as the 2'-C of the deoxyribose sugar. The 5'-C can form a phosphodiester
bond with the previous nucleotide in the chain, but
the 3'-C cannot form
a bond with next incoming dNTP. Addition of a ddNTP
during DNA replication therefore stops synthesis.